1. Field of the Invention
The present invention relates to a laminated piezoelectric component, particularly to a laminated piezoelectric component for use with oscillators and filters.
2. Description of the Related Art
Conventional piezoelectric components include piezoelectric ceramic substrates having vibrating electrodes provided on a surface thereof. The vibrating electrodes are sealed by sandwiching them between two cover substrates via adhesives. Various adhesives have been used depending upon the structure and characteristics of the piezoelectric component and the bonding strength of the adhesive. The thickness of the adhesive is relatively small and the specific thickness is determined based upon the sealing property of the vibration space and prevention of breakage of external electrodes.
When the piezoelectric component is subjected to a high temperature heat treatment, such as reflowing, the crystalline structure of a portion of the piezoelectric ceramic substrate is altered by the heat of reflowing, thereby a resonance frequency Fo (the center frequency of a piezoelectric filter, or the oscillation frequency of an oscillator) differ before and after reflowing. Accordingly, a correction should be provided to account for the change of the resonance frequency Fo due to the heat of reflowing, otherwise the resonance frequency Fo of the piezoelectric component after reflowing varies greatly.
In order to overcome the problems described above, preferred embodiments of the present invention provide a laminated piezoelectric component which minimizes the degree of change of the resonance frequency before and after being subjected to a high temperature heat treatment, such as reflowing, and further minimizes the distribution of the resonance frequency.
Preferred embodiments of the present invention provide a laminated piezoelectric component having piezoelectric substrates on the surface of which vibration electrodes are provided, cover substrates defining a laminated body together with the piezoelectric substrates, and adhesives located between the piezoelectric substrates and the cover substrates to bond the piezoelectric substrates to the cover substrates, wherein the elasticity modulus and thickness of the adhesive are determined so that a change in the resonance frequency due to the heat applied to the piezoelectric substrate is canceled by the change in the resonance frequency caused by a synthesized stress that is imparted on the piezoelectric substrate from the cover substrate and adhesive.
The resonance frequency described herein refers to a center frequency in the case of a piezoelectric filter and the oscillation frequency in the case of an oscillator.
The synthesized stress imparted on the piezoelectric substrate from the cover substrate and adhesive is controlled by the modulus of elasticity and thickness of the adhesive. Practically, the modulus of elasticity c and the thickness b2 of the adhesive are determined such that the calculated value using the following equation is approximately zero:
a2xc3x97b2xe2x88x92a3xc3x97b3xe2x88x92a1xc3x97b1xc3x97exp[xe2x88x92(b2/c2)xc3x97107]
where a1 (/xc2x0C.) and b1 (xcexcm) denote the thermal expansion coefficient and thickness of the cover substrate, respectively, a2 (/xc2x0C.), b2 (xcexcm) and c (MPa) denote the thermal expansion coefficient, thickness and modulus of elasticity of the adhesive, respectively, and a3 (/xc2x0C.) and b3 (xcexcm) denote the thermal expansion coefficient and thickness of the piezoelectric substrate.
Consequently, the change of the resonance frequency due to the synthesized stress imparted on the piezoelectric substrate from the cover substrate and adhesive cancels the change of the resonance frequency due to the:heat imparted on the piezoelectric substrate. As a result, the degree of change of the resonance frequency before and after a :high temperature heat treatment, such as reflowing, is minimized.
The stress imparted on the piezoelectric substrate from the cover substrate is absorbed by the adhesive in the laminated piezoelectric component according to preferred embodiments of the present invention. In other words, the modulus of elasticity c and the thickness b2 of the adhesive are determined such that the synthesized stress imparted on the piezoelectric substrate includes only the stress component imparted on the piezoelectric substrate from the adhesive. Actually, the value of (b2/c2), which is a coefficient of exp[xe2x88x92(b2/c2)xc3x97107], is determined to be 5.0xc3x9710xe2x88x927 or more in the equation of a2xc3x97b2xe2x88x92a3xc3x97b3xe2x88x92a1xc3x97b1xc3x97exp[xe2x88x92(b2/c2)xc3x97107]. This unique arrangement eliminates the influence of the cover substrate, thus allowing only a synthesized stress having a given amount of stress from the adhesive to be imparted on the piezoelectric substrate. Accordingly, the degree of change of the resonance frequency before and after a high temperature heat treatment, such as reflowing, is greatly reduced and minimized. Further, the distribution of the resonance frequency is greatly reduced and minimized.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the drawings attached hereto.